Method and apparatus for measuring solderability of metal surfaces

ABSTRACT

A method of obtaining an indication as to the solderability of a metal consists of focussing an optical telescope on a surface of molten solder in a container and noting the vertical height of the telescope, dipping a sample of the metal to be soldered into the molten solder, adjusting the vertical height of the telescope and refocussing it at the top of the solder meniscus formed up the sample, noting the difference between the two heights of the telescope and comparing the change in height with the change in heights achieved for samples of good solderability and poor solderability.

States Patent [1 1 Monger et al.

[11] 3,76Lii [451 Sept. 25, 1973 METHOD AND APPARATUS FOR MEASURINGSOLDERABILITY or METAL SURFACES [75] Inventors: Kenneth Sydney Monger,Great Baddow; Frank Wallis, Chelmsford, both of England [73] Assignee:The Marconi Company Limited,

Chelmsford, Essex, England [22] Filed: Dec. 6, 1972 [21] Appl. No.:312,748

[30] Foreign Application Priority Data Dec. 11, 1971 Great Britain:57,656/71 [52] U.S. Cl 356/156, 29/484, 228/56, 228/565, 228/57 [51]Int. Cl 823k 37/04 [58] Field of Search 356/156, 249; 228/56, 56.5, 57

[56] References Cited OTHER PUBLICATIONS Tests for Tin Lead SolderedJoints Bureau of Mines Report No. 6963 US. Dept. of Interior June 1967.

Solderability Testing, W. B. Harding, Bendix Corp. Kansas City Division,Conf. 650702-2, Cont. N0. AT(- 29-1 )6l3.

Solders and Soldering, H. H. Manko, McGraw-I-Iill USAEC (1965) N.Y.,1964, pp. 262 -272.

Primary Examiner-David Schonberg Assistant ExaminerS. K. MorrisonAttorney-Donald M. Wight et al.

[ 5 7] ABSTRACT A method of obtaining an indication as to thesolderability of a metal consists of focussing an optical telescope on asurface of molten solder in a container and noting the vertical heightof the telescope, dipping a sample of the metal to be soldered into themolten solder, adjusting the vertical height of the telescope andrefocussing it at the top of the solder meniscus formed up the sample,noting the difference between the two heights of the telescope andcomparing the change in height with the change in heights achieved forsamples of good solderability and poor solderability.

11 Claims, 4 Drawing Figures l I l PATENTEB SEPZSIQTS SHEET 1 BF 2PATENTEDSEPZSIQB 7 1, 1

SHEET 2 BF 2 SAMPLE FLUX Y= HEIGHT 0F MEN/SCUS SOLDER F /G.Z I

7 SOLDER/ FLUX INTERFAC/AL TENS/0N 7 =450d/cm.

METHOD AND APPARATUS FOR MEASURING SOLDERABILITY OF METAL SURFACES Thisinvention relates to apparatus for measuring the solderability ofmetallic surfaces.

The important factor in making satisfactory soldered joints is theability of the two surfaces to be joined to be wetted by molten solder,or in other words the solderability of the surfaces. In 'order to avoidwaste, it is desirable in many cases to determine the solderability ofthe parts to be soldered before assembly proper. It is common to checkthe solderability of round wire, for example, by what is known as thebead test. The round wire is fluxed and lowered into a globule of moltensolder. The time which elapses between the wire contacting the globuleandthe solder flowing round the wire is an indication of the wiressolderability. For flat samples, it is common to use the sample to skimthe surface of a solder bath or to dip the sample vertically into thesolder bath for varying times. In this case an indication ofsolderability is provided by'the minimum time of exposure of the sampleto the solder for acceptable wetting to occur.

Whilst the above mentioned methods provide a useful rough guide to thesolderability of the sample involved, no measure is given of how good orthorough the wetting is.

Apparatuses are available by which a measure of the goodness andthoroughness of wetting may be obtained. Such apparatuses usually relyon measuring the wetting force exerted-on the samples during verticaldipping. Not only are such apparatuses complicated, relatively delicateand expensive, but also the results obtained can be subject toconsiderable error.

The present invention seeks to provide improved methods of measuring thesolderability of surfaces and improved apparatuses for carrying out suchmeasurements, which are realtively simple yet capable of providingusefully reliable indications of solderability.

According to this invention, a method of obtaining an indication of thesolderability of a metallic surface comprises the steps of noting thelevel of molten solder in a container, taking a sample of the metal tobe soldered and comparing the height of the meniscus formed up thesample with the previously noted level of the solder to obtain anindication of the solderability of said metallic surface.

Normally the sample would be fluxed prior to being lowered into saidmolten solder.

As will be appreciated, if the sample is of substantial volume comparedwith the volume of solder in said container, the general level of thesolder will rise as said sample is lowered therein to. If, however, thesample is of very small volume compared with the volume of solder insaid container, the rise in solder level due to the volume of the sampleentering therein may be neglected. In the case where a metal sheet isrequired to be tested for solderability, it would be common to take asample of the strip, 0.375 inch wide and of thickness 0.016 inch andlower this into a cylindrical container of solder 3 inches in diameterand 3 inches deep.

According to a feature of this invention an apparatus for obtaining anindication of the solderability of a metallic surface comprises acontainer for containing a quantity of molten solder, means formeasuring the level of solder within said container, means for lower.-ing a sample of the metal to be soldered into said solder and means formeasuring the height of the meniscus of solder up said sample.Preferably said means for measuring the level of solder in saidcontainer and the height of the meniscus comprises a microscopeincorporating a sight line and mounted to be of adjustable height bymeans of a'vernier control. Preferably again the means for lowering saidsample includes means for controlling the rate of descent of said sampleinto said solder. This last mentioned means may comprise a peg adaptedto abut an abutment on a member passing through a beam, arranged abovesaid solder container, means being provided for suspending said samplefrom said member, the abutment of said member having a hole providedtherein adapted to accept said peg when said member is rotated to alignsaid hole and said peg. Said hole may be an open hole extending throughsaid abutment, control of the rate of descent of said sample beingobtained by virtue of the friction between said peg and the sides ofsaid hole, or said hole may be a blind hole, control of the rate ofdescent of said sample being obtained by the leaky piston and cylindereffect between said peg and said blind hole or said hole may be a blindhole with an air bleed passageway so dimensioned as to achieve arequired rate of descent of said sample.

In order to facilitate initial measurement of the level of solder insaid container, preferably a rod is provided having along its length anadjustable abutment, said rod and abutment being provided to enter thehole in said beam from which said member passes, until said abutmentabuts said beam, whereupon by adjustment of said adjustable abutment theend of said rod directed towards solder contained in operation in saidcontainer, may be adjusted until this almost touches the surface of saidsolder, whereby the sight line of said microscope may be aligned midwaybetween the end of said rod and the reflection of that end in the sufaceof said solder. Preferably the end of said rod adjacent said solder ispointed. Conveniently said container is electrically heatable.v

The invention is illustrated in and further described with reference tothe accompanying drawings in which,

FIG. l is a perspective view of one apparatus for obtainingan-indication of the solderability of a metal sample in accordance withthe present invention FIGS. 2 and 3 are explanatory graphical diagramsand FIG. '4 shows a modification to the apparatus of FIG. ll. Likereferences in FIG. d denote like parts in FIG. I.

Referring to FIG. 1, an electrically heated solder container l is filledwith solder 2. The container 1 is mounted upon a base 3 from whichextend three vertical members, two referenced 4 at what is in operationthe front of the apparatus and one referenced 5, at the rear of theapparatus. The vertical members 4 and 5 support a top beam 6 whichextends over the solder container ll. A sample holding clip 7 issuspended by a shaft 8, from a member 9 which member 9 is slidable in avertical direction within a hole lit) in the top beam 6 over the surfaceof the solder 2. The member 9 is prevented from passing completelythrough the hole 10 by an abutment 12. A hole 13 extends through theabutment l2 and is such as to accept a peg 114 upstanding from said beam6, when said member 9 is rotated to the position shown. The peg 14 isprovided not only to hold the member 9 in an extreme upper position inwhich a sample held within the clip 7 is clear of the solder 2, butalso, by virtue of the fact that it is a relatively close fit within thehole 13, acts to cause th member 9 to fall relatively slowly uponalignment of the peg l4 and the hole 13.

An optical microscope 15 having a sight line 16 in corporated therein ismounted in a carriage 17. The carriage 17 is mounted on the verticalmembers 4 so as to be movable in a vertical direction. Movement of thecarriage 17 is effected by means of a spring 11 and a shaft 18 driven bya vernier or micrometer head 19. Accessory to the apparatus so fardescribed is a rod 20 having an adjustable abutment 21. The rod 20 andthe abutment 21 are such that the rod may be passed through the hole inbeam 6 so far as the abutment 21, which abutment 21 may then be adjusteduntil the pointed end 22 of the rod 20 almost touches the surface of thesolder 2.

In operation the surface of the molten solder 2 in container 1 isskimmed to clear any dross. The rod 20 is passed through hole 10 andabutment 21 adjusted until the pointed end 22 of rod 20 almost touchesthe surface of the molten solder 2. The operator then adjusts thevertical position of microscope until the sight line 16 appearsequidistant between the real end 22 of the rod and the reflection of thesame in the surface of the solder. The reading of vernier or micrometerhead 19 is then noted. The rod 20 is then removed and the member 9complete with rod 8 and sample clip 7 is then inserted through the hole10 and the abutment l2 allowed to rest against the peg 14. A sample isthen inserted in sample clip 7 and the member 9 is rotated until the pegl4 and the hole 13 are aligned. The member 9 is then allowed to fall.The friction between the peg 14 and the walls of hole 13 ensuring thatthe sample enters the solder sufficiently slowly as to ensure that therise of solder up the sample is due to surface tension rather thanthrough any disturbance of the surface of the solder 2. The microscope15 is then raised and the sight line 16 aligned with the top of themeniscus of solder up the sample. The reading of ver nier or micrometer19 is again noted, the difference between this last reading and thefirst mentioned reading providing an indication of the solderability ofthe sample. This indication may be interpretted by comparing the effectachieved with a sample of known good solderability and with the effectachieved with a sample of poor or bearly acceptable solderability. It isbelieved, however, that the meniscus height gives a direct indication ofthe quality of wetting as is explained with reference to FIGS. 2 and 3.

Referring to FIGS. 2 and 3 the most widely accepted criterion ofwetting, and hence solderability, is the contact angle between theliquid and the solid sample. Height of rise of meniscus is related tothe contact angle, see FIG. 2, by the following expression:

0 is the contact angle g is the acceleration due to gravity p is thespecific gravity of the solder p is the specific gravity of the flux 7is the tension in the interface between solder and Y is the height ofrise of the meniscus g, p and p are fairly well established constants; yhas been found to lie in the region between 350 and 450 dynes percentimetre for 63/37 tin-lead solder and water-white resin flux at 260CFor this solder and flux system therefore, the relationship between Yand6 is as depicted in FIG. 3.

The present applicants believe that the measurement of meniscus height,Y, can be converted directly to quality of wetting in terms of contactangle 0, subject only to some doubt as to the true value of theinterfacial tension 7, For samples so far tested the above describedrange of values for y is found to be valid. Regardless of doubt howeverthe apparatus may be used purely to provide comparative testing.

Referring to FIG. 4 in the modification shown therein the hole 13 inabutment 10 is blind. The hole 13 and the peg 14 in this case actinglike a leaking cylinder and piston to achieve a slow rate of descent ofthe member 9.

We claim:

1. A method of obtaining an indication of the solderability ofa metallicsurface comprising the steps of noting the level of molten solder in acontainer, dipping a sample of the metallic surface to be soldered intothe solder and noting and comparing the height of the meniscus formed upthe sample with the previously noted level of the solder whereby thecomparison provides an indication of the solderability of said metallicsurface.

2. A method as claimed in claim 1 and wherein the sample is fluxed priorto being lowered into said molten solder.

3. An apparatus for obtaining an indication of the solderability of ametallic surface comprising a container, a quantity of molten soldercontained in the container, height measuring means for measuring thelevel of solder within said container, :1 sample of the metal to besoldered, lowering means for lowering said sample into said solder andheight measuring means for measuring the height of the meniscus ofsolder up said sample.

4. An apparatus as calimed in claim 3 and wherein said height measuringmeans for measuring the level of solder in said container and the heightof the meniscus comprises a microscope incorporating a sight line andmounted to be of adjustable height by means of a vernier control.

5. An apparatus as claimed in claim 4 and wherein the lowering means forlowering said sample includes means for controlling the rate of descentof said sample into said solder.

6. An apparatus as claimed in claim 5 and wherein said last mentionedmeans comprise a peg adapted to abut an abutment on a member passingthrough a beam, arranged bove said solder container, means beingprovided for suspending said sample from said member, the abutment ofsaid member having a hole provided therein adapted to accept said pegwhen said member is rotated to align said hole and said peg.

7. An apparatus as claimed in claim 6 and wherein said hole is an openhole extending through said abutment, control of the rate of descent ofsaid sample being obtained by virtue of the friction between said pegand the sides of said hole.

8. An apparatus as claimed in claim 6 and wherein said hole is a blindhole, control of the rate of descent of said sample being obtained bythe leaky piston and cylinder effect between said peg and said blindhole.

9. An apparatus as claimed in claim 6 and wherein said hole is a blindhole with an air bleed passage-way operation in said container, may beadjusted until this almost touches the surface of said solder, wherebythe sight line of said microscope may be aligned midway between the endof said rod and reflection of that end in the surface of said solder.

1.1. An apparatus as claimed in claim 10 and wherein the end of said rodadjacent said solder is pointed.

1. A method of obtaining an indication of the solderability of ametallic surface comprising the steps of noting the level of moltensolder in a container, dipping a sample of the metallic surface to besoldered into the solder and noting and comparing the height of themeniscus formed up the sample with the previously noted level of thesolder whereby the comparison provides an indication of thesolderability of said metallic surface.
 2. A method as claimed in claim1 and wherein the sample is fluxed prior to being lowered into saidmolten solder.
 3. An apparatus for obtaining an indication of thesolderability of a metallic surface comprising a container, a quantityof molten solder contained in the container, height measuring means formeasuring the level of solder within said container, a sample of themetal to be soldered, lowering means for lowering said sample into saidsolder and height measuring means for measuring the height of themeniscus of solder up said sample.
 4. An apparatus as calimed in claim 3and wherein said height measuring means for measuring the level ofsolder in said container and the height of the meniscus comprises amicroscope incorporating a sight line and mounted to be of adjustableheight by means of a vernier control.
 5. An apparatus as claimed inclaim 4 and wherein the lowering means for lowering said sample includesmeans for controlling the rate of descent of said sample into saidsolder.
 6. An apparatus as claimed in claim 5 and wherein said lastmentioned means comprise a peg adapted to abut an abutment on a memberpassing through a beam, arranged bove said solder container, means beingprovided for suspending said sample from said member, the abutment ofsaid member having a hole provided therein adapted to accept said pegwhen said member is rotated to align said hole and said peg.
 7. Anapparatus as claimed in claim 6 and wherein said hole is an open holeextending through said abutment, control of the rate of descent of saidsample being obtained by virtue of the friction between said peg and thesides of said hole.
 8. An apparatus as claimed in claim 6 and whereinsaid hole is a blind hole, control of the rate of descent of said samplebeing obtained by the leaky piston and cylinder effect between said pegand said blind hole.
 9. An apparatus as claimed in claim 6 and whereinsaid hole is a blind hole with an air bleed passage-way so dimensionedas to achieve a required rate of descent of said sample.
 10. Anapparatus as claimed in claim 6 and wherein a rod is provided havingalong its length an adjustable abutment, said rod and abutment beingprovided to enter the hole in said beam from which said member passes,until said abutment abuts said beam, whereupon by adjustment of saidadjustable abutment the end of said rod directed towards soldercontained in operation in said container, may be adjusted until thisalmost touches the surface of said solder, whereby the sight line ofsaid microscope may be aligned midway between the end of said rod andreflection of that end in the surface of said solder.
 11. An apparatusas claimed in claim 10 and wherein the end of said rod adjacent saidsolder is pointed.